Luther has been well established as a first name in English speaking countries in honor of MartinLuther for quite some time.

The surname Luther is itself derived from given names either Luither or Lothar. (Hans Bahlow's book on German surnames says it's hard to distinguish between the two origins in terms of modern surnames.) One of the given name dictionaries I have from Germany, Walter Burkart's Neues Lexikon der Vornamen, does give Luther as a variant form of Lothar.

But is Luther actually used as a first name in modern Germany? If parents in Germany wanted to name a son Luther, would they be allowed to, or would it be prohibited under the law in Germany that prevents surnames from being used as given names? How do modern Germans react when they hear of Americans with Luther as a first name, such as Luther Vandross or Luther Burbank?